Former oil minister Diezani Alison-Madueke is under investigation for allegedly trying to rig last year’s elections, a spokesman for the country’s anti-corruption agency said on Tuesday.

“We are investigating a case involving the former petroleum minister Mrs Alison-Madueke in respect of some millions of dollars she is accused of giving to INEC officials to influence the 2015 elections,” Wilson Uwujaren, of the Economic and Financial Crimes Commission (EFCC), told AFP.

Local media has claimed the Independent National Electoral Commission (INEC) received $115 million in oil money to influence the polls in favour of then president Goodluck Jonathan’s People’s Democratic Party (PDP). Alison-Madueke was petroleum minister in Jonathan’s government until last year, when he was defeated by Muhammadu Buhari Some suspects, including senior INEC officials, local oil company executives and the head of a commercial bank through which the bribes were alleged to have been paid, have been arrested.

The EFCC has also frozen the bank accounts of those implicated, Uwujaren added. The 55-year-old former oil minister has been repeatedly targeted by a range of corruption claims, which she has flatly denied. In October 2015 she was arrested in London, where she was undergoing breast cancer treatment, as part of a British probe into international corruption and money laundering.

She has also been accused of wasting millions of dollars on private jets while many Nigerians live below the poverty line. Buhari, who took office in May last year, has vowed to end endemic corruption in Nigeria, which is Africa’s largest oil producer and biggest economy.

He has begun an overhaul of the state-run oil firm as part of a wider crackdown on graft that he said has seen “mind-boggling” sums of public money looted over decades. Alison-Madueke was in charge of the sector when there were claims the Nigerian National Petroleum Corporation (NNPC) failed to remit $20 billion in oil revenue to the central bank.